1. Field of the Invention
The present invention relates to a diagnosis circuit for a potentiometer, particularly in the areas of a potentiometer, where undefined states occur.
2. Description of the Background Art
Potentiometers are known to have, among other things, a carbon track (resistance track), along which a slider is guided to pick up a signal, and the track thereby has a track beginning and a track end, and thus has an area, over which, in the case of a single-turn potentiometer, the slider moves. When the slider makes contact with the carbon track, a defined contact resistance occurs which can be described as a linear curve. Only at the end of the path, or the beginning of the path does the contact resistance occasionally deviate to substantially higher values due to a minimal contact to the substrate. This increased contact resistance results in indefinable voltage values at the leap from start to end values at the transition point (blind spot). Practical experience has shown that in a subsequent AD (digital and analogous) transducer, an averaged value is registered, which the software interprets as a normal value, and which can deviate considerably. For this reason, for example, no unequivocal reading of a set value can be defined, which changes the reading at random. Frequently, this area is therefore circuit-technically masked out and is not utilized, or is positioned between two stop positions.
In DE 199 55 461 A1, a potentiometer of this class is described, which, together with a servomotor, is used as a servo device for a powered operation of baffles etc., for example. It is mentioned in the description that a small rotational angle of approximately 4 to 8 degrees, which occurs in single-turn potentiometers, is compensated for by electronics. However, no further details are provided.
From DE 197 38 050 A1, a circuit for generating and evaluating the measurement signals of a position sensor is known to identify errors which occur because the contact resistance between the resistor core and the pickup frequently changes in an undefined manner to a relatively large degree. The position sensor, essentially an adjustable resistor, is operated at a constant current to obtain a useful signal and a test or plausibility signal. For this purpose, the partial voltages caused by the partial resistances of the position sensor and the contact resistance between the pickup and the resistor core are registered, from which a useful signal, which depends on the position of the pickup, but which is unaffected by the contact resistance, as well as a test signal that is dependent from the contact resistance, is generated. That is, the supply currents of the potentiometer are utilized in the evaluation.
EP 0 354 269 B1 discloses a circuit arrangement for monitoring the contact resistance of a potentiometer serving as a position transmitter. Based on the fact that contact resistances can form between the resistance track and the slider moving thereupon, which alter the original assignment of slider position and voltage level, an increase in the contact resistance is already registered when a safety-critical level has not been reached yet. In a control device, a current sensing resistor, among other things, is inserted between a terminal for the supply voltage for the potentiometer and an OP amplifier, the voltage drop of which can be fed via an input to a microprocessor as a representative signal for the current flowing via the parallel connection from the potentiometer and an adjusting resistor.
The two last-mentioned solutions deal with calculating the influence of the contact resistance of the potentiometer in an adjustment range within the boundaries of the substrate, that is, by recognizing when it is too big.